1. Field of the Invention
The present invention relates to a method of manufacturing sheets having superplastic properties from a nickel-based superalloy known by the name of "alloy 718", and also to a method of forming parts by superplastic deformation of such sheets.
2. Summary of the Prior Art
Numerous applications of superplastic forming processes have been developed, particularly in aeronautics, for producing parts from titanium and aluminium based sheets. Some sandwich-type structures in particular are obtained by combining the processes of superplastic forming and diffusion welding. These applications have demonstrated the advantages, in terms of quality, cost, productivity and industrial expertise, of using a superplastic forming process for making parts from sheets.
In addition, the designer's preferred choice of material for numerous parts of casing or fixed structure type for aircraft engines, turbojets or turbo-machines is a nickel-based superalloy known by the name of "alloy 718", and a typical composition of which is, by weight, 19% Cr, 18% Fe, 5% Nb+Ta, 3% Mo, 1% Ti, 0.5% Al, and mainly Ni as the remainder.
Currently, however, sheets of alloy 718 do not possess superplastic properties, and mechanical elongation characteristics as low as A%=12 are normally allowed for these sheets.
Consequently, the invention seeks to determine particular manufacturing and forming conditions which will enable parts to be made from sheets of alloy 718 by superplastic forming.